Monocytes and macrophages function in a myriad of host-defense mechanisms. The spectrum of monocyte functions has become increasingly important and the heterogeneity among monocytes and macrophages has become more defined. It is not known whether the different monocyte functions are carried out by genetically restricted subsets, analogous to T cells and B cells of lymphocytes, or whether a multipotential single monocyte population is capable of functional differentiation in response to appropriated regulatory stimuli. We have recently isolated two subsets of human peripheral blood monocytes by elutriation centrifugation. While they share some properties, they differ with respect to others. We have defined the conditions of Percoll density gradient centrifugation under which the elutriation product can be purified further to yield small monocytes in 80% purity. The small, rather than the large, monocyte population in the peripheral blood is the effector cells that are capable of native cytotoxicity against tumor target cells. We have demonstrated that this function is clearly distinct from that ascribable to large granular lymphocytes. Moreover, we have demonstrated that exposure to high concentrations (10%) DMSO can induce responsiveness to lyphopolysaceride and lymphocine in the large monocyte population and render them cytotoxic to herterologic tumor cells. Both large and small monocytes are equally active in antibody-dependent cellular cytoxicity. The purified subsets have been used as immunogens, and murine monoclonal antibodies are being developed to subsets-specific antigens. To date, cell surface antigenic heterogeneity has been defined in both large and small monocytes indicating the likelihood of yet another order of definable, structural, heterogeneity among human peripheral blood monocytes. We have developed an animal model using guinea pig peripheral blood and have isolated three subsets of monocytes, two of which are analogous to the subsets found in peripheral blood. The third appears to be a more differentiated cell with some characteristics of a macrophage. Further exploitation of the guinea pig model will define the kinetics of monocyte macrophage production in the study stage using in vivo labeling technique. Preliminary data indicate there are distinct differences in kinetics of appearance of peripheral blood monocytes that suggest one population does not "mature" from the other. The guinea pig subsets have been used as immunogens in order to raise murine monoclonal antibodies with a high degree of specificity for each subset. When those antibodies are obtained and defined, they will be used to track the maturation, differentiation, and origin in the bone marrow of the subsets of guinea pig monocytes. Our studies have been expanded to include in vitro monocyte macrophage production from bone marrow cells in the steady stage and after specific in vivo probation by defined inflammatory stimuli. The objective of these experiments is to determine whether monocytes arrive from one or more than one definable stem cell. (MB)